Various types of molding operations employ a mold comprising two or more die parts which are movably mounted relative to each other to permit release of a molded part after the molding operation has been completed. Obviously, it is necessary to assure that the die parts are closed prior to introducing fluent molding material into the mold in order to avoid waste, etc.
The problem of assuring that the die parts are closed prior to introducing fluent molding material into the mold is particularly important in connection with automated systems such as an injection molding machine, especially where the fluent molding material, e.g., metal, plastic, etc. has been heated to a temperature which poses a severe health hazard to operating personnel in the event that the material contacts the operator's skin. In connection with an injection molding machine, the material is injected at relatively high pressure into a multi-piece mold immediately following closure of various die parts which form the mold. In the event that the molding material is injected into the mold shortly before closure of the die parts, the relatively high injection pressure causes the material to shoot out from around openings in the die parts and may be carried yards away from machine. As a result of this serious health hazard, metal guards are often installed on the machine so as to protectively surround the mold and thereby shield the machine's operator from errant molding material in the event that injection takes place before the die parts are fully closed.
In the past, others have resorted to the use of a mechanical limit switch for sensing the point in time at which the die parts of the mold are fully closed. In the case of an injection molding machine, one die part remains stationary while the other die part is reciprocated from an open, release position to a closed molding position by means of a piston and cylinder; a limit switch is mounted on the machine so as to be tripped by the movable die part approximately at the point where the die parts make contact. This approach to the problem has not been satisfactory from the standpoint of reliability. The mechanical limit switch and trip arm employed to actuate the same are subject to wear and misadjustment. More importantly, in connection with molding operations employing heated molds and molding material, the various parts of the mold and machinery expand (due to heating) relative to the trip arms and other parts employed to mount the switch, thereby altering the point at which the switch is actuated. Consequently, it may be appreciated that the use of prior art limit switches provides, at best, a means of sensing the gross position of the die parts relative to each other.
Others in the past have suggested the use of a pneumatic system for sensing the closure of two parts. For example, U.S. Pat. No. 4,106,607 issued Aug. 5, 1978 to Charles Badavas discloses a pneumatic safety system for a safety enclosure surrounding a machine. The enclosure includes a door which, when opened, stops operation of the machine to prevent injury to personnel. The system disclosed in this prior patent is similar to that previously described in the sense that it merely provides for gross position sensing and is not adaptable for use with an injection molding machine wherein it is necessary to precisely correlate injection timing with the complete closing of the die parts.
Thus, there is a clear need in the art for a system which reliably senses the complete closure of two or more die parts, whose accuracy is not adversely affected by the temperature of the mold and which delays enablement of the injection process is a fixed time following closure of the die parts to assure that the injection occurs only after the mold is closed and pressure has built up in a pressurized fluid system which closes the parts.